1. Field of the Invention
The present invention relates to an electronic circuit breaker, and relates particularly to a circuit breaker in which a current signal may be used to simulate an overload current when testing the circuit breaker to insure that the solid state trip circuit is functioning correctly.
2. Prior Art
FIG. 2 is a block diagram showing a general configuration of a conventional circuit breaker 5 whose tripping characteristics are shown in FIG. 3. The operation-inhibiting circuit 25 opens the contacts 30 to inhibit a tripping operation if the power supply voltages are too low for the respective circuits in the circuit breaker to operate normally. Normally, the output contacts 30 are closed by the operation-inhibiting circuit 25. When any one of the tripping circuits 18-21 outputs a trip signal, the trip signal is supplied through the contact 30 to the electronic switch 8, which is closed by the trip signal, to energize the trip coil 3. The energized trip coil 3 then drives the tripping mechanism to open the contacts 2 of the main circuit. The trip signal is also supplied to the ALM circuit 9 which outputs an alarm signal. The trip signals from respective tripping circuits 18-21 are directed to the short-time/instantaneous tripping display circuit 24, the long-time tripping display circuit 23, and the ground overcurrent tripping display circuit, respectively.
While a circuit breaker using a current transformer as a current sensor suffers from the saturation of a magnetic core at higher currents, a circuit breaker using an air coil as a current sensor has the advantage that the output of the sensor is not saturated. In the conventional air coil type circuit breaker, a voltage is developed across the air coil. Thus, it is necessary to feed a voltage test signal from an external testing device when testing the function of the circuit breaker in the field. However, it is a current-injecting type testing device that a workman usually uses in the field.